Conventionally, there is known a copying machine or a multi-function machine mounted with an image scanning device for scanning an image recorded on a sheet. The image scanning device is usually of a type called flat-bed scanner (FBS) including: a platen glass on which a sheet (document) is placed; and an image sensor unit that is disposed under the platen glass so as to move back and forth in parallel to the glass plane. See JP-A-11-168592 for example. The image sensor unit may have a structure in which an image sensor, such as a CCD (Charge Coupled Device) or a CIS (Contact Image Sensor), is mounted on a retention member, such as a carriage.
In the image sensor unit described in JP-A-11-168592, an image sensor having a rectangular solid shaped case is mounted on a retention member. The image sensor unit scans an image from an object to be scanned, such as a sheet or a document, in two directions configured as a primary scanning direction and a secondary scanning direction. The primary scanning direction is configured to be in parallel to a longitudinal direction of the image sensor unit, and the secondary scanning direction is configured to be in perpendicular to the longitudinal direction.
The case of the image sensor has a pair of rotation shafts having a common axis extending parallel to the longitudinal direction. The rotation shafts are provided on surfaces of a pair of concave portions (insertion portions) that are provided at positions slightly inward from both end faces of the case in the longitudinal direction. The rotation shafts are inserted into rotation holes (rotation bearings) formed in the retention member. The image sensor is thus rotatably supported by the retention member.
When assembling the image sensor and the retention member, the positioning of the image sensor relative to the retention member is performed by engaging a protruded portion, which is provided in the retention member and has a width, with a groove portion, which is provided in the case of the image sensor and has a width in the longitudinal direction.
A procedure of forming the image sensor unit by assembling the image sensor and the retention member will be briefly described in below.
First, the end of each rotation shaft of the image sensor and the corresponding rotation hole of the retention member are disposed opposite to each other, and then the rotation shaft is inserted and fitted into the rotation hole. At this time, the groove portion of the image sensor and the protruded portion of the retention member are positioned to engage with each other.
Next, when the retention member is rotated with respect to the image sensor, the protruded portion of the retention member engages with the groove portion of the image sensor, and thus the image sensor is positioned relative to the retention member in the primary scanning direction, thereby forming the image sensor unit.
In a so-called flat-bed type image scanning device, the longitudinal direction of the case is configured as the primary scanning direction and the transverse direction of the case is configured as a secondary scanning direction. The image sensor unit scans image while being moved in the secondary scanning direction relative to an object to be scanned (hereinafter, simply referred to as a “scanned original”).
When performing the scan, a movement distance or the image sensor unit in the secondary scanning direction is equal to or greater than the length of the scanned original in the secondary scanning direction. A maximum movement distance of the image sensor unit in the secondary scanning direction is equal to or greater than a length obtained by adding a length of the image sensor unit in the transverse direction and the length of the scanned original having a maximum size, which is scannable by the image scanning device mounted with the image sensor unit, in the secondary scanning direction.
Accordingly, a size of the image scanning device in the secondary scanning direction becomes equal to or greater than the maximum movement distance of the image sensor unit in the secondary scanning direction.
However, in the conventional image sensor unit as described in JP-A-11-168592, the groove portion and the protruded portion positioning the image sensor relative to the retention member in the primary scanning direction are provided at the above-mentioned positions of the image sensor and the retention member, respectively. Accordingly, the length of the image sensor unit in the transverse direction includes the length of positioning members (the groove portion and the protruded portion) in the secondary scanning direction.
Since a light emitting portion and a light receiving portion are arranged in the longitudinal direction of the case of the image sensor, the image sensor needs to have the length in the secondary scanning direction for accommodating the light emitting portion and the light receiving portion as well as the necessary length for the positioning members.
Accordingly, the image sensor unit requires a certain amount of size in the secondary scanning direction, and thus the image scanning device requires certain amount of size in the secondary scanning direction.
Generally, there is known that, when a gap between an upper surface of the image sensor and a bottom surface of the platen glass varies, the distance between the upper surface of the image sensor and the scanned original placed on the platen glass also varies, and thus, the image cannot be satisfactorily scanned from the scanned original. This problem becomes more apparent if the image sensor unit uses a CIS for the image sensor, because CIS has a relatively small focal depth. Accordingly, it is necessary to position the upper surface of the image sensor with respect to the bottom surface of the platen glass with high accuracy, in order to maintain the gap, between the upper surface of the image sensor and the bottom surface of the platen glass, constant.
However, in the conventional image sensor unit as described in JP-A-11-168592, the shaft provided in the image sensor is inserted and fitted into the rotation hole (rotation bearing) provided in the retention member. Therefore, the image sensor is not configured to be movable at its rotation shafts in vertical direction with respect to the retention member. Accordingly, when a gap between the bottom surface of the platen glass and the retention member varies, the gap between the upper surface of the image sensor and the bottom surface of the platen glass varies accordingly, and thus the image sensor becomes unable to satisfactorily scan an image from the scanned original.
In the conventional image sensor unit as described in JP-A-11-168592, since the image sensor is rotatably held by the retention member, and the axis of the rotation is being configured near one end in the transverse direction of the case of the image sensor, the rotation with a very small angle largely varies the gap between the upper surface of the image sensor and the bottom surface of the platen glass. Thus, the image sensor may not satisfactorily scan the image from the scanned original.